This paper describes the development and application of flash x-ray radiography for studying the core structure of atomizing jets at high temperatures (to 2300 K) and pressures (1.4 - 13.8 MPa). Methyl iodide (CH3I) was injected at approximately 75 m/s through a 1 mm circular orifice into (hot) nitrogen, or the post combustion products of an H2/Air/Ar mixture. Injections at 40 m/s through a thin annular orifice into these environments were also conducted. Radiographs which resolve core details to less than 0.1 mm were achieved. We observed that the core structure is strongly affected by heat transfer under these conditions. The core structures of circular jets were found to develop helical structures with wavelengths longer than the jets diameter, and the core penetrations decreased for denser ambient gas. A surprising result was that for the same gas density, the lengths of evaporating cores decreased with increase in pressure up to a "critical" value, then increased with further rise in pressure. The minimum corresponds to a pressure and temperature near the thermodynamic critical point of the liquid.